Work on bovine pyruvate kinase from skeletal muscle, erythrocytes, and brain has resulted in highly purified muscle and erthrocyte enzyme while resulting in a partial purification of sixty-fold in the cerebral enzyme. Both the skeletal and aethrocyte pyruvate kinases are inhibited greatly by sulfate ion. This inhibition is not a reflection of the cation activation or inhibition generally known for this enzyme. Cerebral enzyme has not yet been tested. The brain of cows seems to be an excellent source for the M isozyme of pyrivate kinase with large amounts of high activity enzyme found in the 50-70 percent ammonium sulfate cut. These purified preparations are to be used in studies on the physical parameters of the isozymes and the interactions of these isozymes with kinetic modulators. Physical studies are being conducted on the scanning gel chromatograph.